神經壞死病毒感染誘發之細胞介素-1β及腫瘤壞死因子-α對神經元死亡的影響

Abstract

神經壞死病毒(NNV)是病毒性神經壞死症的病原，造成了包括石斑魚在內的許多海水魚幼苗大量死亡及巨大經濟損失。病毒性神經壞死症的病理特徵是大腦與視網膜壞死及空泡化。本研究室先前發現，受到NNV感染而垂死的石斑魚大腦中，細胞介素-1β (interleukin-1β, IL-1β)等促發炎細胞激素的基因表現量很高。為了解促發炎細胞激素是否在NNV感染造成魚腦神經細胞死亡的過程中扮演重要角色，本研究首先建立石斑魚大腦之初代培養細胞為研究系統，再感染NNV。結果發現，NNV感染會促進微小膠細胞(microglia)的增生以及神經細胞的死亡，且神經細胞壞死的比例隨NNV感染劑量的升高而增加。初代培養的腦細胞在感染NNV之後，只有神經細胞及微小膠細胞中可以測到病毒鞘蛋白，星狀膠細胞則無感受性。在初代培養的腦細胞感染NNV三天後所收集的細胞上清液(Culture supernatant, CS)中，可測到由微小膠細胞分泌的兩種前發炎細胞激素，IL-1β 及腫瘤壞死因子-α (Tumor necrosis factor-α, TNF-α)。收集CS，先以專一性抗體中和CS中的NNV，然後將此上清液加入新的初代培養腦細胞中，結果與加入未中和病毒的CS組一樣，都會造成神經細胞的減損。將CS先以IL-1β抗血清中和其活性後，再感染初代培養細胞，對神經細胞的毒性會比未處理組弱；當NNV感染劑量相同時，含有IL-1β重組蛋白的NNV病毒液會比不含IL-1β重組蛋白的病毒液造成更多神經細胞死亡，對微小膠細胞的增生效果也更明顯。因此推論，NNV感染後會誘發腦中微小膠細胞產生IL-1β，且IL-1β對神經細胞的死亡扮演著重要的角色。

Nervous necrosis virus (NNV), a non-enveloped virion with two single-stranded positive-sense RNAs, belongs to betanodavirus genus of Nodaviridae family. NNV is the causative agent of viral nervous necrosis (VNN) disease, which has caused mass mortality and economic loss in many species of cultured marine fish, including groupers (Epinephelus spp.). The pathological characteristic of VNN is the vacuolization and necrosis of brain and retina. Our team has found that the expression level of interleukin (IL)-1β gene became very high in the brain of moribund groupers with VNN. Whether IL-1β cytokine played a critical role in the neuronal death of NNV-infected groupers is unclear, and is clarified in this study. The primary tissue culture of grouper brain (pGB cells) was established, and was infected with NNV. NNV infection caused the proliferation of microglia and death of neurons in pGB cells, and the amount of dead neurons was viral load-dependent. Viral capsid protein was detected in the neuron and microglia, but not in the astrocytes. The microglia in pGB cells released IL-1β and tumor necrosis factor (TNF)-α in the CS at 3 days post infection (dpi). NNV-specific polyclonal antibodies were used to neutralize NNV. The CS collected at 3 dpi, treated with or without NNV-specific polyclonal antibodies, was separately added into two sets of new pGB cells, and both were able to induce neuronal death in pGB cells. Also, the neurotoxic effect in CS-treated pGB cells was attenuated by pre-adding anti-IL-1β pAb in the CS. Moreover, treatment of pGB cells with combination of NNV and rIL-1β resulted in more dead neurons, compared to sole infection of NNV. We therefore suggested that the IL-1β secreted by NNV-infected pGB cells play an important role for the induction of neuronal death and IL-1β in CS was the substance for neurotoxic effect that caused more serious neuronal death after NNV infection.